I have asked just everyone if their is a fast way to find the top 32-bits of a 32-bit x 32-bit multiply? There were multiply instructions that returned all 64 bits but they take 17 or 18 cycles doing something not used so:MULSHIFT32: lsrs r3, r0, #16 //Factor0 hi [16:31] uxth r0, r0 //Factor0 lo [0:15] uxth r2, r1 //Factor1 lo [0:15] lsrs r1, r1, #16 //Factor1 hi [16:31]
muls r0, r1 //Factor0 lo * Factor1 hi muls r2, r3 //Factor1 lo * Factor0 hi muls r1, r3 //Factor1 hi * Factor0 hi
adds r0, r2 //(Factor0 lo * Factor1 hi) + (Factor1 lo * Factor0 hi)
movs r2, #0 // adcs r2, r2 //C --> bit 16 (r2 contains $00000000 or $00010000) lsls r2, r2, #16 //
lsrs r3, r0, #16 //Extract partial result [bits 16-31]
adds r2, r3 //Partial [bits 16-47] adds r1, r2 //Results [bit 32-63]Now the problem I have is that I cannot find my copy of the red book (Joseph Yiu's book on programming the M0 & M0+).The fact that it currently takes 4 instructions to move C into bit 16 of a register looks like it MAY be possible to speed up so that rather than two ADDS at the end, a single ADDS Rd, Rn, Rm since all registers are low.So, now we are getting somewhere. I should add that my good friend Sarah Avory wrote the logic in C and simply tested it with every possible value to check it was correct. She was also able to save a cycle which seems tiny by todays standards, but in certain applications, the MULSHIFT32 is used millions of times a second.
Sean Dunlevy said:Sarah Avory
Ha, she seems to be a 6502 addict :-) Please guide her to the Lynx, she'd love this machine.
She wrote a Lynx game. The PC Engine was the best 6502-based console. Sarah is coding C64 and PS5 at the same time. YES, she is a crazy lady.
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